Electron microscope



Nov. 21, 1944, s. RAMO ELECTRON MICROSCOPE Fired sept. 11, 1942 2Sheets-Sheet l k. Q A :nw m .ba r HR L@ e JM Mwave 1m SAW n: b

Nov. Z1, 1944. s, RAMO ELECTRON MICROSCOPE Filed Sept. l1, 1942 2Sheets-Sheet 2 Inventor Simon Ramo,

by JVM/175W Hls Attorheg.

Patented Nov. 21, 1944 ELECTRON MICROSCOPE Simon Ramo, Schenectady, N.Y., assigner to General Electric Company, a corporation of New YorkApplication September 11, 1942, Serial No. 457,952

(Cl. Z50-49.5)

19 Claims.

The present invention relates to improvements in electron microscopes.

This application is a continuation-impart of my copending @applicationlSerial No. 391,380, filed May 1, 1941, and which is assigned to theassignee of the present application.

'I'he electron microscope may be described briefly as an apparatus forproducing by electronic means a magnified image of an object desired tobe examined. In such apparatus, the image-producing functions performedby light `in optical microscopy are accomplished in a somewhat analogousmanner by a stream of electrons.

Since the electron microscope possesses theoretically enormous resolvingpower it promises to be an extremely valuable instrument :for scientificinvestigation. However, the types of electron microscope now availableare very much limited in application because of their structuralcomplexity and resultant high cost.

In order to provide a simpler form of microscope it has been suggestedto make use of divergent rays of electrons produced from a finelypointed cathode having its tip in close proximity to the object to beexamined. By projecting these rays through the object, it is possible toobtain an enlarged shadow replica of the object without the use of acomplex electron lens system. From a practical standpoint, however, thistype of microscope has also proven of limited utility for the reasonthat its use at voltages suiciently high to assure penetration of theobject by the electrons tends to produce very rapid deterioration of theemitting surface of the cathode. Such microscopes have beencharacterized by large size and relatively high evacuation, the highvacuum necessitating elaborate evacuating equipment which is bulky,expensive, and time-consuming in operation. Ordinarily, in order toavoid arcing between the electrodes at operating voltages it has beenfound necessary to evacuate the tubes to pressures of the order of 0.01micron.

From a practical standpoint the operating voltage necessary to providean image of suitable intensity is fixed by the necessity for impartingto the electrons a velocity sucient to penetrate the specimen. At thepresent time voltages of the order of 25,000 to 50,000 are regarded assuitable for satisfactory image reproduction. At such voltages positiveion action has heretofore imposed drastic limitations upon spacing ofelements Within the tube and the necessary degree of evacuation. Inelectron microscopes provided -approached as thlimitin hcros'cope u esmay be substantiallyareducedsin with electrostatic focusing lenses, ithas been necessary to maintain substantial spacing of the lenselectrodes and a high degree of evacuation in order to render the meanfree path of electrons sufficiently great to prevent the formation ofpositive ions in suicient quantity to cause arcing over at the lensesand destructive bombardment of the screen. High vacuum has beennecessitated also by the need for maximizing the mean free path ofpositive ions further to preclude ionization of the residual gas. In theshadow type of electron microscope, which utilizes a point source ofelectrons, satisfactory operation above 10,000 volts has heretofore beenunobtainable even with the usual spacing and vacuum lbecause of the factthat the effectiveness of the iinely pointed cathode is very quicklydestroyed by positive ion bombardment at higher voltages.

If positive ion bombardment can be substantially prevented, it isevident that all types of electron microscopes, both the lens and theshadow types, can be reduced in size and the degree of evacuation withinthe limitations imposed by the mean free path of electrons. By the sametoken higher voltages would be permissible in either the shadow type orlens type of microscope at conventional spacing and pressure. Heretoforethe limitations imposed by the mean free path of electrons has not beeneven remotely approached, since at vacuums ordinarily maintained, thispath is of the order of meters. Thus if the electron free path agghe,

factor electron msize and operated at a relatively high internal gaspressure, i. e., poor vacuum sufficient only to render the electron meanfree path of the same or at least as great as the desiredsourceto-screen distance within the tube.

Accordingly, one aspect of my invention, which relates to shadow-typemicroscopes of the character just referred to, consists in the provisionof means for preventing destruction of the cathode while neverthelesspermitting the examination of objects of such density as to require highvoltage. In this connection, an important feature of the inventionconsists in the use with the microscope of an energizing circuit of suchchai'- acter as to limit the application of voltage to intervals of lessthan about 10"G seconds. By

this means instantaneous electron currents of sucient penetration andintensity to permit the production of useful images are realized, whileat the same time effects tending to produce deterioration of the cathodeare minimized.

Accordingly, it is a general ob'ject of my invention to eliminatepositive ion action as the limitation upon gas pressure and electrodespacing in electron discharge apparatus, thereby to permit structureslimited only by the length of im It is a further objec of my inven ionto provide an electron microscope having a substantially reducedsource-to-screen length which is capable of satisfactory imagereproduction in a relatively poorly evacuated chamber.

Further, and specific objects of my invention reside in the provision ofmeans for reducing the source-to-screen length of an electron microscopeto the same order of magnitude as the length of the mean free path ofthe imaging electrons, and also for permitting satisfactory operation ofsuch a device with internal gas pressures sufficiently high to causedestructive positive ion action in electron microscopes heretoforeknown.

It is still another object of my invention to provide means forprecluding cathode destruction in shadow type electron microscopes whenoperating at practical image producing voltages.

My invention also contemplates means for operating well known types ofelectronI microscopes at high voltages heretofore thought.to bedestructive.

According to my invention, positive ion action is substantiallyeliminated and the accompanying space, pressure and voltage limitationsheretofore mentioned in connection with existing electron microscopesare thereby removed by using in connection with the microscope anenergize ing circuit of such character as to limit the application ofvoltage to intervals of the order of -6 seconds (that is, Within orclose to the range from 10-3 to 10`9 seconds). By this meanssubstantially instantaneous electron currents of sufficient penetrationand intensity to permit the production of useful images are realized,while at the same time eiects tending to produce lens sparkover anddeterioration of the cathode and screen are substantially avoided.

For a better understanding of my invention, reference may be had to thefollowing description taken in connection with the accompanyingdrawings, and its scopewill be pointed out in the appended claims. Fig.1 is a diagrammatic representation of an electron microscope andassociated circuit embodying certain aspects of my invention; Fig, 2 isan enlarged fragmentary view of one of the elements shown in Fig. 1;Fig. 3 illustrates an alternative embodiment of my invention; Fig. 4diagrammatlcally illustrates a still further embodiment of my inventionrelating to an electron microscope and ass-ociated energizing circuits,and Fig. 5 illustrates a modied form of the electron microscope suitablefor connection to the energizing circuit shown in Fig. 1.

Referring particularly to Fig. 1, there is illustrated a dischargevessel I 0, assumed to be of highly evacuated character and shown partlybroken away. At one end of the vessel is provided an image-reproducingsurfacewlhh which may consist, fo'e'anmllmf'altronresponsive uorescentscreen, or, alternatively, of an electron- :photographic v ,ilm. At theother eiid of the vessel, there is mounted a pointed non-thermioniccathode in the form of a metal body I2, suitably of tungsten. Thecathode has a smoothly rouilcgetglg (shown greatly magnied in ig. 2),which is formed, for example, by etchin and the radius of curvature ofwhich. (indicated by 1') is preferablylless than about 10-3 centimeters.In order to eliminate the possibility of undesired emission from thelateral surfaces of the cathode it may be surrounded by a metal shieldI5 which is maintained at cathode potential.

In cooperative relation with the cathode there is provided anotherelectrode I'I which is shown as an arinular metal disk, and which isadapted to establish an electron-accelerating field in theI vicinity ofthe cathode. The sharply pointed\ character of the cathode makes itpossible to establish very intense elds at the cathode tip f by theapplication of readily attainable poten- Y) tials to the electrode IIand by this means to, obtain cold cathode emission from the tip. I

Due to the symmetry of the electrode I'I, electrons proceeding from thecathode tip may be expected to follow divergent lines extending from thecenter of curvature of the tip toward the image-reproducing surface l I.In order to make use of this fact an object desired to be investigated(for instance, a bacteriological specimen) is positioned in closeproximity to the extremity of the cathode. This may be done, forexample, by supporting the specimen in the central aperture of theelectrode H as indicated ttt-T9 or, alternatively, by the provision ofan obJeEt mount (not shown) which is structurally inde,- pendent of theelectrode I1.

If the cathode tip is in Very close proximity to the object underinvestigation, and if the potentials involved are suicient to assurepenetration of the object by the emitted electrons, a magnied shadowpicture of the object Will'X be projected on the image-reproducingsurface, j the image produced being characterized by vary- 1 ing degreesof light and shadow depending upon local variations in density orthickness of the J object. The magnification which can be obf tained inthis way is a direct function of the ratio of the distance between thecathode and the reproducing surface to the distance between the cathodeand the object and may readily be made Very great.

Unfortunately, it is found in practice that the small dimensions towhich the emitting surface of the cathode is required to be limitedseriously restricts the permissible operating conditions of theapparatus. In particular, it is observed that unless the operatingpotential is maintained below about 10,000 v-olts, rapid deteriorationof the cathode occurs. This is believed to be due in part to bombardmentof the cathode by ions of the residual gas in the discharge space and inpart to excessive heating of the cathode tip as a resultmf therelatively large current drawn from it. xIn some cases, the heating ofthe cathode due to these causes may become so great as to result in theestablishment of a destructive arc supported by evolution of metal vaporfrom the cathode itself. Because of these effects, the cathode quicklydeteriorates to a condition in Iwhich it is unsuitable for its intendeduse. Inasmuch as most of the objects which are amenable to investigationby the electron microscope require for their penetration electrons ofvelocity corresponding to a potential eld greatly in excess of 10,000volts, it will be seen that the considerations just stated apparentlypreclude any widespread use of the shadow microscope.

In accordance with the present invention, the limitations implied in theforegoing are removed by the provision of means for preventingdeterioration of the cathode point while at the same time permittingoperation at voltages of the order of 50,000 volts or higher. This isaccomplished, as will be explained more fully in the following, by theuse of an energizing system of such character as to limit theapplication of voltage to intervals of less than about one micro-second.By this means ion bombardment is substantially avoided and heating ofthe cathode due to this and other causes is restricted to a safe(non-destructive) value. V

The energizing system shown in Fig. l comprises a circuit which isoperative to apply very short pulses of voltage between the cathode I2and the electrode I'I. In this connection voltage is derived from thesecondary of a transformer 22 which is assumed to have its primarywinding connected to an alternating current supply source 23, thetransformer being preferably chosen to lprovide a voltage on the orderof or greater than 50,000 volts.

The voltage thus obtained is impressed on a condenser 24 which has oneterminal connected to the electrode I'I through ground. The otherterminal of the condenser connects with an element 26 comprising onemain electrode of a spark gap 2l', the other main electrode of the gapbeing indicated at 28. A conductor 30 connected between the latterelectrode and the cathode I2 assures that the voltage across condenser24 shall be applied between the cathode and the grounded electrode I'Iwhenever the gap 2l becomes conductive.

The gap 2'I should be of such dimensions as normally to sustain withoutbreakdown any voltage apt to be impressed across it during its intendeduse. However, there is provided in connection with it an auxiliaryelectrode 32 adapted, when appropriately energized, to permit sparkoverof the gap, assuming, of course, that the voltage across the gap isfavorable to such an occurrence. The auxiliary electrode 32 is connectedto a triggering circuit which enables it to be subjected to voltagepulses tending to cause r such sparkover. In the arrangementillustrated, the triggering circuit includes a so-called peakingtransformer 34 which possesses a readily saturable magnetic circuit ofsuch character as to facilitate the production of sharp pulses ofvoltage in the transformer secondary 35. (The theory and construction oftransformers of this type are fully described in B. D. Bedford PatentNo. 1,918,173, granted July l1, 1933.) The primary winding 36 of thetransformer is connected to the supply source 23 so that the pulses ofvoltage induced in its secondary winding are synchronized with thevariation of the potential developed across the condenser -24 by thetransformer 22. A saturating winding 3l, excited from a direct currentsource 38, permits the phase relationship between the voltage pulsesdeveloped in the winding and the voltage across the condenser 24 to beadjusted within close limits.

The circuit connected with the electrode 32 is preferably so adjusted asto trigger the spark gap 21 at a time when the terminal of the condenser24 which is connected to the electrode 2E approaches maximum negativepotential. Under these conditions breakdown of the gap obviouslyimpresses full condenser potential between the cathode I2 and theelectrode II, thus initiating a cold cathode discharge from the former.The electrons produced in this way from the cathode tip will, because ofthe high potential at which they are created, penetrate the object I9and produce a magnified shadow image of the object on the surface I I inaccordance with principles previously stated.

In order to limit the duration of the discharge to a period sufficientlyshort to avoid excessive heating of the cathode and consequentdestruction of the cathode tip, provision is made for terminating thedischarge very soon after the initiation. The means employed in thisconnection comprises in the arrangement shown a second spark gap 40having main electrodes 4I and 42 and a discharge controlling auxiliaryelectrode 43. The energization of the electroderli is preferablycorrelated to that of the electrode 32 to assure that the gap 40 shallbe triggered within less than a micro-second after the breakdown of thegap 2. This is accomplished by energizing the electrode 43 from atriggering circuit consisting of a second peaking transformer 44 whichis identical with the transformer 34 and which is supplied from the samesource. However, in circuit with lthe transformer secondary winding 45there is included a time delay circuit including a resistor 4'I and acondenser 48. If the values of the elements 41 and 48 are properlychosen, the voltage pulses impressed on the electrode 43 will f beslightly delayed with respect to the pulses impressed on the electrode32. As has been previously indicated, it is desired for present purposesthat this delay should be on the order of a micro-second or less.Sparkover of the gap 40 obviously has the effect of grounding thecathode I2 and of making the continuation bf a discharge from itimpossible.

Restoration of both spark gaps to a condition of non-conductivity willbe accomplished in due course by passage through zero of the potentialderived from the transformer 22. For this reason, it is possible toobtain a cyclically renewed discharge through the electron microscope ata frequency corresponding to the frequency of the source to which thetransformer 22 is connected. The total current flow in each dischargecycle may be regulated by the use of a series resistor 39 which limitsthe rate of charging of the condenser 24 and consequently restricts theamount of charge available for dissipation during the discharge period.

With this mode of operation, and assuming the duration of eachconductive interval of the vention therefore provides a means forextending the field of useful application of the shadow microscope tothe examination of objects requiring high velocity electrons for theirpenetration.

It will be understood that the spark gaps 2l and 40 may be replaced byother means for providing controlled conductivity. For example, one mayuse, in lieu of the gaps, controlled gaseous discharge tubes such asthyratrons.

A further modilication of the invention is shown in Fig. 3, whichdiffers from the construction previously described mainly in theprovision of means for permitting the use of a shorter dischargechamber. (In this figure, elements which have been previously describedin connection with Fig. 1 are identified by the same index numerals.)

As has been previously pointed out, the magnication obtainable with theshadow microscope depends upon the ratio of the distance between thecathode tip and the image-reproducing surface to the distance maintainedbetween the cathode tip and the object under investigation. Insituations which require the maintenance of a relatively large gapbetween the cathode and the object (for example, due to the nature ofthe object) the length of the discharge space required to yieldsatisfactory magniiication may obviously be quite great. In suchsituations, in order to avoid the use of an unduly elongated structure,one may provide an electron lens system adapted to produce in theimage-reproducing plane a replica of the crosssectional pattern of theelectron stream at a point at which the stream has already attainedsubstantial divergence.

This is illustrated in Fig. 3 by the use of a series of annular lenselements Ila, llb and I 1c adapted to provide a lens system of shortfocal length. The potentials of the various lens elements are maintainedmost conveniently in appropriate relationship by connecting the elementsto terminals of the impulse voltage source. With the arrangement shown,the lens may be expected to focus the electron beam as indicated by thedotted lines B and B', it being assumed that the imaging plane of thelens corresponds to the surface Il.

By this means, with a discharge space of given length, an image ofgreater magnification may be obtained than would be realized merely byrelying upon the unfocused divergence of the electron rays proceedingfrom the cathode. Moreover, in view of the fact that the lens system is,in effect, concerned solely with the magniiication of a virtual image ofappreciable size (as distinguished from the object itself), it does notrequire to be of high resolving power, and its design may be carried outwithout reference to those complicating factors which make theconstruction of a microscope dependent wholly upon lens action sodiicult. The result is, therefore, that the use of a lens system in thisconnection does not sacrifice the inherent advantages of the shadow typemicroscope with respect to simplicity of design, low cost, etc.

Referring now to Fig. 4 of the drawings, there is illustrated anotherembodiment of my invention comprising a discharge Vessel 49 formed of ahollow metallic body portion 50 and a metallic end closure 5| suitablysealed together by an intermediate section of insulating material suchas a glass annulus 52. It will be understood that the discharge vessel49 may also suitably be formed entirely of glass or other ceramicinsulating material. The discharge vessel or envelope 49 may beevacuated by suitable pumping means 49a arranged to maintain within theenvelope any desired pressure of residual gas,

which may be air. The metallic end portion 5I provides support for aninwardly extending cathode 53. The form of the cathode 53 may differ inaccordance with the type of electron microscope which is desired. Forexample, in the focusing lens type of electron microscope the cathode 53may be of any Well known thermionic type comprising an electricallyheated electronemitting filament 53a disposed within an apertureddirecting shield 53h, as shown at Fig. 5. On the other hand, in a shadowtype electron microscope it is necessary to provide a very nely pointedcathode of the non-thermionic type, suitably of tungsten. The pointedtype of cathode is shown at Fig. 4 and comprises a smoothly rounded tip(shown greatly magnied at Fig. 2) which is formed, for example, byetching, and the radius of curvature of which (indicated by r) ispreferably less than 103 centimeters. In order to eliminate thepossibility of undesired emission from the lateral surfaces of thecathode it may be surrounded by a metal shield 5l a which is maintainedat cathode potential and may be formed integrally with the end plate 5|.At the end of the vessel 49 opposite the cathode 53 there is provided animage-reproducing surface comprising a glass window 54 coated internallywith an electron-responsive fluorescent screen 55. It will be understoodthat, if desired, the screen 55 may be replaced by a removableelectron-sensitive photographic film.

In cooperative relation with the cathode 53 for' the production of ashadow image there is provided another electrode 56 which is shown as anannular metal disk and which is adapted to establish an electronaccelerating field in the vicinity of the cathode. The sharply pointedcharacter of the cathode makes it possible to establish very intenseelds at the cathode tip by the application of readily attainablepotentials to the electrode 5B and by this means to obtainl cold cathodeemission from the tip. The electrode 56 may be used also to position anobject to bev investigated (for example a bacterlological specimen) inclose proximity to the extremity of the cathode. This may be done, forexample, by supporting the specimen in the central aperture of theelectrode 56 as indicated at 51. It will be understood that, if desired,an object mount structurally independent of the electrode 56 may beprovided.

If a pointed cathode tip is used, as indicated in the drawings, and ifthe tip is in very close proximity to the object under investigation,electrons will be drawn from the point source at the end of the cathodeand, due to the specified shape of the tip, may be expected to followdivergent lines extending from the cathode tip toward theimage-reproducing surface 55. In following this path the electronspenetrate the object '51 and project upon the image-reproducing surfacea magnied shadow picture of the object, the image being characterized byvarying degrees of light and shadow depending upon local variations indensity or thickness of the object. Without further renement only shadowmagnication can be obtained in this way, and the degree of magnificationpossible is a direct function of the ratio of the distance between thecathode and the reproducing surface to the distance between the cathodeand the object.

For the purpose of increasing the magnication possible with apredetermined source-toscreen distance, I provide an electrostatic lenssystem comprising a series of metallic disks B- 64, inclusive, each ofwhich is centrally apertured to permit the passage of theimage-producing electrons. Alternate disks, for example, the disks 59,6I and B3, are mounted directly upon the metallic container 50 and aremaintained at ground potential. The intermediate disks, namely the disks58, 60, 62 and B4, are mounted upon a common electrically conductingsupport B5 which is electrically connected to the end plate 5l andmaintained at cathode potential. The metallic disks are suitablyseparated by insulating spacers 66. In the shadow type of electronmicroscope, the lens system comprising the charged disks 58-64,inclusive, serves only to produce a wider divergence of the electronstream emanating from the point source at the tip of the cathode 53 andthus to produce a larger image upon the surface 55 for a predeterminedsource-to-screen length of the discharge vessel 49.

Fig, 1, may be dispensed with.

It will be understood that the electron microscope illustrated in thedrawings may serve either as a shadow type of microscope or as a lenstype and that my invention is equally applicable to both types. If thecathode 53 has a finely pointed tip and the electrode -56 is in closeproximity to the tip, the microscope will function as a shadow type, theelectrostatic lens system serving to increase the divergence of theimaging electrons. On the other hand, if the cathode 53 does not act asa point source, due either to destruction of the tip or substitution ofa thermionic type of cathode, the microscope may be used as a focusingtype if the object-to-lens and lens-to-screen spacing is arranged inaccordance with the well known general optical equation iifrr where Aequals the distance from the object to the one of the so-calledprincipal planes of the lens, B equals the distance from the otherprincipal plane of the lens to the screen, and F is the focal length ofthe lens.

In order to minimize the size of the electron microscope described abovethe last stages of in position to view the electron-produced image( onthe screen 55.

Electron microscopes of the high vacuum type which are currently in useare commonly limited to a minimum lens element spacing of approximatelyone quarter of an inch at 50,000 volts and internal pressures of theorder of 0.01 micron. Such spacing results in a minimum focal length ofone quarter of an inch per lens, and a minimum source-to-screen lengthof approximately twelve inches for satisfactory magnification even whenused in conjunction with a light microscope. According to my invention Ipropose to space apart the lens elements 58-64 by distances of the orderof 50 mils at 50,000 volts so that a great many lenses may be positionedwithin a relatively short vessel. With such spacing to provide aplurality of lenses of short focal length, satisfactory magnication maybe obtained with a source-to-screen length of the order of two to fourinches. It will be evident that for operation at 25,000 volts, which issuicient for the tainable will be so small that the limitations will bepurely mechanical, while a source-to-screen length of as little as oneinch does not appear unreasonable.

My invention also comprehends that a microscope of the above dimensionsshall be operable with a significant amount of residual gas present,that is, under such poor vacuum conditions as have heretofore causedlens sparkover and other adverse positive ion effects in electronmicroscopes even with wide element spacing. By way of example, adischarge tube having an internal pressure of greater than 0.1 micronmay be regarded as including a significant quantity of gas. Conversely,it is contemplated that by retaining conventional spacing and pressuresmy invention will make it possible to operate a microscope at greatlyincreased voltages thereby to obtain greater object penetration.Furthermore, the same means-relied-upon to provide the above innovationsalso precludes cathode tip destruction in the shadow type microscopehaving a pointed tip, even when operated at upward of 25,000 volts in apoor vacuum. These improved results are simultaneously attained, as willbe explained more fully in the following, by the use of an energizingsystem, similar to that explained above with respect to Figs. 1 and 3,to limit the application of voltage to intervals within or approachingthe range from a millisecond to millimicrosecond (10-3 to l09 seconds).By this means positive ion bombardment is substantially eliminated sothat heating and destruction of the cathode destructive bombardment ofthe screen, and arcover between the lens electrodes are all avoided.

The circuit for applying a pulse of voltage to the cathode structure orend plate 5| of the arrangement of Fig. 4 may be similar to that shownin connection with Fig. 1 and corresponding elements have been assignedlike reference numerals.

It is emphasized that this portion of the circuit comprising the secondspark gap 40, having main electrodes 4l and 42 and a dischargecontrolling auxiliary electrode 43, serves to limit the duration of thedischarge during a period suiiiciently short to avoid adverse positiveion effects of all types, and for terminating the discharge very soonafter its initiation. The energization of electrode 43 is preferablycorrelated to that of the electrode 32 to assure that the gap 4i] shallbe triggered within a time of the order of a microsecond (l0-3 to 199seconds) after the breakdown of the gap 2l. This is accomplished byenergizing electrode 43 from a triggering circuit comprising the peakingtransformer 44 and the associated circuit. If the values of the elements41 and 48 are properly chosen and correlated, the voltage pulsesimpressed on electrode 43 will be slightly delayed with respect to thepulses impressed on electrode 52. Sparkover of the gap 40 obviously hasthe eiect of grounding the cathode 5i and of making the continuation ofa discharge from it impossible.

With a pulse discharge of the type described above and assuming theduration of each conductive interval of the microscope to beapproximately within the range from 10-3 to 109 secgreat majority ofobjects, the lens spacing at- 'l5 onds, the destructive and spacelimiting eiects of positive ion action are substantially eliminated.While I do not desire to be limited to any particular theoreticalexplanation, it is my present understanding that the markedly improvedresults obtainable with my pulse electron microscope are attributableprimarily to the fact that potential pulses of the order of timeduratio-n described prevent any positive ions present within thedischarge tube from ever attaining any substantial velocity. It isgenerally understood that the mass and consequently the inertia of apositive ion is substantially greater than that of an electron.Consequently, while electrons may be emitted from the cathode anddischarged upon the image-reproducing surface during each shortconductive pulse, a pulse of the order of magnitude described is notsuciently long to produce 'any substantial movement of a heavierpositive ion or to provide it with any significant velocity.Subsequently, when the conductive pulse ceases, the ion quickly comes torest and must again be accelerated to standstill by the next pulse.Accordingly, for all practical purposes the positive ions present in thedischarge tube may be considered as stationary. In this way any positiveion action is substantially eliminated, since adverse positive ioneffects are believed to be due largely to bombardment of the tubeelements and the residual gas molecules by positive ions moving at asignificant velocity.

Since the positive ions in my electron microscope are substantiallystationary in space for all practical purposes, that is, they neverattain a sufficient velocity to have a signicant effect, the microscopeis substantially independent of the degree of evacuation so far aspositive ion eiects are concerned. Furthermore, the structurallimitations formerly imposed upon electron microscopes b-y positive ionbombardment are removed. There is no mean free path of effectivelystationary positive ions, so that tube element spacing and evacuationare not controlled by this factor. Instead there is substituted as alimitation only the mean free path of electrons. This path issufliciently large even under poor vacuum conditions to permit spacingof tube elements as close as mechanically possible. Indeed, it is evenpossible to retain clarity of image production to a point where thesource-to-screen length of the tube approaches the mean free path ofelectrons. Then a tube of predetermined length need be evacuated only tomeet the limitation, and conversely, a tube having a predeterminedvacuum must have a length determined -by the mean free path of electronsat such pressure. It will be understood that if the mean free path ofelectrons is less than the source-to-screen distance, the averageimaging electron will encounter an obstacle before it reaches thescreen, so that the images will not be clear. The relatively stationarycharacter of the positive ions also removes voltage limitationsheretofore imposed, so that microscopes having greatly improvedpenetrating power become possible by utilizing Voltage of the order ofhundreds of thousands of volts.

While I have shown and described my invention as applied to a particularsystem embodying various devices diagrammatically shown, it will beobvious to those skilled in the art that changes and modifications maybe made without departing from my invention, and I, therefore, aim inthe appended claims to cover all such changes and modifications as fallWithin the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electron microscope comprising an evacuated envelope whichcontains a cathode having a non-thermionic electron-emitting portion inthe form of a finely rounded point, means for sup- Iporting in closeproximity to the said point an object desired to be examined, anelectrode adapted when at high potential to draw electrons from the saidpoint along divergent lines extending through the region of support ofthe object, and means for applying between the electrode and the cathodepulses of voltage of suicient magnitude to assure penetration of theobject by the electrons thus obtained from the cathode, said pulsesbeing limited in duration to less than about 10"-6 seconds, wherebyexcessive deterioration of the cathode point is prevented.

2. In Vacuum discharge apparatus for producing in a selected imagingplane a magnified electron-optical image of an object desired to beinvestigated, the combination which includes a cathode having anon-thermionic electron-emitting portion in the form of a metal pointwith a radius of curvature at the tip of less than 10-3 centimeters,means for supporting the object to be investigated in close |proximityto the cathode point and in a location between the point and the saidimaging plane, and means for producing in the vicinity of the cathodepoint an intense electrostatic field of such strength and direction asto produce electrons from the point by field emission and to projectSuch electrons through the object along divergent lines extending towardthe imaging plane, said last-named means being operable to maintain thesaid eld for time intervals limited to less than 10-6 seconds, wherebythe cathode point is protected from destructive bombardment.

3. In vacuum discharge apparatus for producing in e, selected imagingplane a magnified electon-optical image of an object desired tobeinvestigated, the combination which includes a cathode having anon-thermionic electron-emitting portion in the form of a metal pointwith a radius of curvature at the tip of less than 10-3 centimeters,means for supporting the object to be investigated in close proximity tothe cathode point and in a location between the point and the saidimaging plane, electrode structure capable when at high potential ofproducing in the vicinity of the cathode po-int an electrostatic fieldadapted to project electrons from the point along divergent pathsextending through the object toward the imaging |plane, said structurebeing either inclusive of or in addition tox the said object-supportingmeans, and an impulsing circuit connecting with the said electrodestructure, saidcircuit including means for abruptly applying a highpotential to the structure and for terminating the application of suchpotential in less than about 10-6 seconds.

4. In vacuum discharge apparatus for producing in a selected imagingplane a magnified electron-optical image of an object, desired to beexamined, the combination which includes a cathode having anon-thermionic emitting portion in the form of a finely rounded point,means for supporting the object to be examined in close Iproximity tothe said point, and in a location between the point and the selectedimaging plane, an electrode adapted when at high potential to drawelectrons from the said point along divergent lines extending throughthe object and toward the imaging plane, said electrode being eitherinclusive of or in addition to the said object-supporting means, acircuit for applying between the cathode and the electrode pulses ofvoltage of suiicient magnitude to assure penetration of the object bythe electrons thus obtained from the cathode point, said pulses being ofsuch limited duration as to avoid excessive deterioration of the cathodepoint, and an electron lens system between the said electrode and theimaging :plane for producing in said plane a magnified replica of thecross-section pattern of the divergent, electron stream proceeding fromthe cathode.

5. An electron microscope comprising an evacuated envelope containing aquantity of residual gas, a non-thermionic cathode mounted within saidenvelope and having an electron-emitting portion in the form of a finepoint, means for supporting an object to be examined in close proximityto said point and in a location between said point and a selectedimaging plane, an electrode adapted when at high potential to drawelectrons from said point along divergent lines extending through saidobject and toward said imaging plane, and energizingmeans for applyingbetween said cathode and said electrode periodic pulses o-f Voltage ofsuicient magnitude to ensure penetration of said object by saidelectrons, said pulses being of such short duration as to maintainpositive gas ions formed by the resultant electron discharge effectivelystationary in space thereby to avoid excessive deterioration of saidcathode point by positive ion bombardment.

6. An electron microscope comprising an evacuated envelope containing aquantity of residual gas, a non-thermionic cathode mounted within saidenvelope and having an electron-emitting portion in the form of a nepoint, means for supporting within said envelope an object to beexamined in close proximity to said point and in a location intermediatesaid point and a selected imaging plane, an electrode adapted when athigh potential to draw electrons from said point along divergent linesextending through said object and toward said imaging plane, and meansfor energizing said microscope comprising high frequency pulsegenerating means for periodically applying between said cathode and saidelectrode pulses of voltage of sufficient magnitude to ensurepenetration of said object by said electrons and of a time duration ofthe order of one microsecond.

'7. An electron discharge apparatus comprising an evacuated envelope-containing a quantity of residual gas, a plurality of positive andnegative electrodes positioned in closely spaced relation within saidenvelope, and energizing means for establishing an electron dischargewithin said envelope comprising means for applying between saidelectrodes pulses of voltage of such short duration that positive gasions resulting from said discharge do not attain a suicient velocity toreach a negative electrode within said envelope during the period ofsaid discharge.

8. An electron discharge apparatus comprising an evacuated envelopecontaining a quantity of residual gas, a plurality of electrodespositioned in closely spaced relation within said envelope, saidelectrodes being spaced apart by distances of the order of 50 mils andsaid envelope containing surfaces which are at a negative operatingpotential with respect to one of said electrodes, and pulse generatingmeans for impressing between said electrodes voltage impulses ofsuiiicient magnitude to establish an electron discharge and of suchshort duration as to preclude movement of positive gas ions generated bysaid discharge to reach negative surfaces within said envelope duringthe period of said discharge.

9. An electron microscope comprising a discharge envelope containing aquantity of residual gas, a plurality 0f positive and negativeelectrodes positioned within said envelope and spaced apart by distancesof the order of 50imils, means for supporting within said envelope anobject desired to be examined, and energizing means for establishing anelectron discharge within said envelope comprising means for impressingbetween said electrodes voltage pulses of sufficient magnitude to ensurepenetration of said object by the discharged electrons and of such shortduration that positive gas ions resulting from said discharge do notattain suicient velocity to reach negative ones of said electrodes.

10. An electron discharge apparatus comprising an evacuated envelopecontaining a significant quantity of residual gas, a plurality ofelectrodes positioned within said envelope, and energizing means forestablishing an electron discharge within said envelope comprising pulsegenerating means for periodically impressing between said electrodesvoltage pulses of such short duration that substantially none of thepositive gas ions formed by said discharge reach any of said electrodes.

l1. An electron discharge apparatus comprising an evacuated envelopecontaining a signicant quantity of residual gas, a plurality of positiveand negative electrodes positioned within said envelope and spaced apartby distances of the order of 50 mils, and energizing means forestablishing an electron discharge Within said envelope comprising meansfor impressing between said electrodes high voltage impulses of suchshort duration that positive gas ions formed by said discharge do notattain suflicient velocity t0 reach negative ones of said electrodes.

12. An electron microscope comprising a discharge envelope -containing aquantity of residual gas, a cathode and at least one other electrodepositioned in closely spaced relation within said envelope, means forsupporting within said envelope an object desired to be examined, andmeans for rendering said cathode electron-emissive comprising means forapplying between said cathode and another electrode pulses of voltage ofsuflicient magnitude to ensure penetration oi said object by electronsemitted by said cathode and of a time duration of the order of onemicrosecond.

13. An electron microscope comprising a discharge envelope containing aquantity of residual gas, a plurality of electrodes positioned withinsaid envelope and spaced apart by distances of the order of 50 mils,means for supporting within said envelope an object desired to beexamined, and energizing means for establishing an electron dischargewithin said envelope comprising pulse generating means for impressingbetween said electrodes pulses of voltage of sufficient magnitude toensure penetration of said object by the discharged electrons and of atime duration falling approximately within the range from 10-3 to 10-9seconds.

14. An electron micros-cope comprising a discharge envelope containing asignificant quantity of residual gas, a plurality of electrodespositioned within said envelope, means for supporting within saidenvelope an object desired to be examined, and energizing means forestablishing an electron discharge within said envelope comprising highfrequency pulse generating means for periodically impressing betweensaid electrodes pulses of voltage of sucient magnitude to ensurepenetration of said object by the discharged electrons and f a timeduration falling approximately Within the range from 10-3 to 10-9seconds, whereby positive gas ions resulting from said discharge areconstrained to remain eifectively stationary in space.

15. An electron microscope comprising a partially evacuated dischargeenvelope containing a significantl quantity of residual gas: atsubstantial pressure, means for supporting within said envelope anobject desired to be examined, an electron emissive cathode and at leastone other electrode positioned Within said envelope and arranged whenenergized to establish a stream of electrons from said cathode throughsaid object, and an electron-sensitive image-reproducing surfacepositioned in the path of said electron stream to provide an enlargedelectronoptical image of said object, said image-reproducing surfacebeing spaced from said cathode a distance of the same order of magnitudeas the mean free path of electrons Within said gas.

16. An electron micros-cope comprising a partially evacuated dischargeenvelope containing a significant quantity of residual gas atsubstantial pressure, means for supporting Within said envelope anobject desired to be examined, an electron emissive cathode and at leastone other electrode positioned in spaced relation Within said envelopeand arranged when energized to establish a stream of electrons directedfrom said cathode through said object, said envelope containing surfaceswhich are at a negative operating potential with respect to said otherelectrode, an electron-sensitive image-reproducing surface in the pathof said electron stream beyond said object thereby lto provide anenlarged electron-optical image of said object upon said surface, saidimage-reproducing surface being spaced from said cathode a distanceequal to less than the same order of magnitude as tbn length of the meanfree path of electrons Within said gas, and means for energizing saidmicroscope comprising pulse generating means arranged to impress betweensaid cathode and another electrode pulses of voltage of suicientmagnitude to ensure penetration of said object by said electrons and ofsufficiently short duration to preclude the acquisition of suiiicientvelocity by positive gas ions generated by the resultant electrondischarge to permit said ions to reach negative surfaces within saidenvelope.

17. An electron microscope comprising a partially evacuated dischargeenvelope containing a significant quantity of residual gas atsubstantial pressure, means for supporting Within said envelope anobject desired to be examined, an electron-emisive cathode positionedWithin said envelope to project toward said object a stream "ofelectrons, an electron-emissive cathode Within said envelope andarranged when energized to establish a stream of electrons directed fromsaid cathode toward said object, an electrostatic lens system arrangedin the path of said stream of electrons and comprising a plurality oflens electrodes spaced apart by a distance of the order of 50 mils, anelectron-sensitive image-reproducing surface positioned in the path ofsaid elec,- tron stream to provide an enlarged electronoptical image ofsaid object upon said surface, said image-reproducing surface beingspaced from said cathode a distance of the same order of magnitude asthe length of the mean free path of electrons Within said gas, and meansfor energizing said cathode without lincurring adverse positive ioneffects comprising high frequency pulse generating means arrangedperiodically to impress upon said cathode pulses of voltage of sufcientmagnitude to ensure penetration of said object by said electrons andhaving a time duration of the order of one microsecond.

18. A high voltage electron discharge apparatus comprising an evacuatedenvelope containinga quantity of residual gas, a plurality of electrodespositioned in spaced relation Within said envelope, and energizing meansfor establishing an electron discharge from one of said electrodescomprising means for impressing between said electrodes high voltagepulses of such short duration that positive gas ions formed by saiddischarge remain effectively stationary in space.

19. A high Voltage electron microscope comprising a discharge envelopevcontaining a quantity of residual gas, a plurality of electrodespositioned in spaced relation Within said envelope, means for supportingwithin said envelope an object desired to be examined, and energizingmeans for establishing an electron discharge of high ob-ject penetratingvalue within said envelope comprising means for impressing between saidelectrodes high voltage pulses having a time duration of the order ofone microsecond.

SIMON RAMO.

